Lack of understanding another person's lack of understanding
Curse of knowledge
The curse of knowledge is a cognitive bias that occurs when an individual, communicating with other individuals, unknowingly assumes that the others have the background to understand. For example, in a classroom setting, teachers have difficulty teaching novices because they cannot put themselves in the position of the student. A brilliant professor might no longer remember the difficulties that a young student encounters when learning a new subject. This curse of knowledge also explains the danger behind thinking about student learning based on what appears best to faculty members, as opposed to what has been verified with students.
https://en.wikipedia.org/wiki/Curse_of_knowledge
The curse of expertise encompasses an expert's inability (at least without further training) to communicate their knowledge effectively to a non-expert.
Here is a Psychology Today article ("The Curse of Expertise" by Sian Beilock) describing the phenomenon:
Experts are called upon to teach those less knowledgeable all the time. Teachers must predict the issues and misconceptions that students will face when learning a new and tricky concept in, say, a physics class. And, baseball coaches must understand the types of problems that a pitcher may encounter when learning to throw a new curve ball. If not, how will the coaches devise the right training techniques to help their pitcher out of trouble? Yet, stepping outside your own point of view and relating to people who have less knowledge and skill is not such an easy task. People with a lot of knowledge are not as good as one might think at doing this.
The research summarized in the article shows that experts at using phones were the least accurate at estimating how long it took new users to learn a phone:
Sales people focused so closely on their own performance and how effortlessly they operated the phone that they had a hard time predicting novices' misconceptions and mistakes. Because of this, sales people were the least accurate predictors of the new users' learning time and failed to predict most of the trouble novices got into.
As the original researcher, P.M. Hinds (1999), explains, the cognitive bias is consistent with the curse of knowledge, which is predicated on not being able to effectively think of oneself in a less expert state. The curse of expertise is specific to not being able to communicate or explain that expertise.
Lots of other sources use this term to describe struggles with communicating expertise to new learners:
"The Science of Smart: Four Secrets To Lift The 'Curse Of Expertise,'" NOVA, 28 Jan. 2013.
Fisher M, Keil FC. "The Curse of Expertise: When More Knowledge Leads to Miscalibrated Explanatory Insight." Cogn Sci., 40.5 (2016), 1251-69. doi: 10.1111/cogs.12280. Epub 2015 Sep 15.
"Assume Teachers Are Victims of the Curse of Expertise and Protect Yourself," section title in Michael W. Wiederman, Study Less, Learn More: The Complete Guide for Busy Students.
Unempathic
The counter-example (Person C who understands person B's difficulties) is displaying empathy
empathy (n) The ability to identify with or understand another's situation or feelings.
If I were writing your sentence, I'd simply say that your person A displays "a lack of empathy" toward's B. It's clear and understandable.
Personally, I've never understood the fascination with squeezing things down to one, rare and hard-to-understand word when two simple ones would get the message across much better, but if you absolutely cannot use the spacebar, you can use the form "unempathic", simply by adding the "removal" prefix un- (as in "uninterested", "unfeeling", "unmoved").
Personally I prefer the "lacking empathy" form, as there are other "un-" words formed from positive roots that carry a hint of being opposite, rather than just lacking (see "unholy", "unfriendly", "uncharitable").
A warning: although the "un-" prefix will make the meaning reasonably clear to the reader, no reputable online dictionary lists this word.
"unempathic" does show up in writing, although far less often than "empathic" (Google ngrams says there's a 100x difference, although it does have instances of "unempathic" dating back to the 1950s)
is there a different word meaning "without empathy"?
Despite it's Greeky-ness, "empathy" is actually quite a new word, and it's not Greek, but German. "Empathy" entered the English language as fancy translation of a German word einfühlen, which was coined by philosopher Rudolf Lotze in the 1850s from a literal translation of the Greek "en" and "pathos" (in, feelings) -- this is why the word "reverted" to "Greek" when translated into English.
einfühlen: to put oneself in someone's position, condition, etc.; to understand something inwardly, to empathize with it
(Lotze must have had a specific meaning in mind to create a new word, because German already had a word for this: nachempfinden )
Unfortunately, none of these three words have a single-word antonym with the sense of being unable to see the world from someone else's perspective.
Ironically, Greek does have a similar-sounding word to "en-pathos", but it means "hatred", which just goes to show that sometimes a word you keep using really doesn't mean what you think it means.
I think there are a few single words that might suggest this, but might not explicitly provide the meaning of someone so knowledgeable they can't understand the position of a novice. YMMV.
All definitions from dictionary.com
Inure: to accusom to hardship, difficulty, pain, etc..; toughen or harden; habituation (usually followed by to)
Habituate: To accustom (a person, the mind, etc.), as to a particular situation
Acclimate: To accustom or become accustomed to a new climate or environment; adapt.
Some example usages:
- Susan had become so inured to the vagaries of a smart phone that she could scarcely understand the difficulties Eric was having.
- Habituated to the reality of US politics, Susan could scarcely understand Eric's confusion.
- While acclimated to 80-hour workweeks, Susan could still empathize (Thanks, @KrisW) with new associates who were not so inured to the demanding schedule.